The invention relates to a device for detecting the weight loaded on a vehicle seat with a sensor and a spring body that connects the vehicle seat to an underbody of a corresponding motor vehicle and is connected, in a first fastening area, with a bottom rail of the vehicle seat on the one hand and, in a second fastening area, to the underbody of the corresponding motor vehicle on the other hand, one of these mounting areas being provided with a bore through which a seat fastening screw is engaged.
Such type detection devices are known and have been described for example in DE 100 55 619 A or DE 100 36 479 A. It is characteristic and substantial for this detection device that it be arranged between bottom rail and underbody. Usually, vehicle seats are detachably fastened to the underbody at four screw connection points by means of four seat fastening screws. The invention only relates to such detection devices of the type mentioned herein above in which the weight of the vehicle seat and of its occupancy, a passenger or a load, is measured in the region of these seat fastening points. At least three, preferably four, detection devices are provided for each vehicle seat. Through the different indications given by these detection devices it is possible to detect whether the vehicle seat is more loaded in its rear region, or more on its left side, and so on. In the event of an accident, it can be detected whether the vehicle seat is loaded or unloaded.
In the detection devices of the type mentioned herein above, the sensor is configured to be a resistance strain gauge and is located on the spring body. The sensor detects the bending of the spring body between the two fastening areas.
These previously known detection devices have proved efficient. The advantage thereof is that spring body and sensor are forming a unit and that they do not need to be separately mounted as a result thereof. They are also very sensitive and are capable of detecting variations of less than one kilogram in the weight loaded onto the vehicle seat.
However, the prior art detection devices also have disadvantages. The effective distance moved by the two fastening areas relative to each other when the weight loaded on the vehicle seat changes, i.e., the deflection and expansion of the spring body, is converted into a considerably smaller change in a region located between the two fastening areas. Since the bending variations are so small, several resistance strain gauges, which are disposed on the two facing main surfaces of the spring body, are usually used.
Another disadvantage is that the spring body must be designed in such a way that it is provided with optimum spring properties in the weight range of concern of for example 0–100 kg while still being highly resistant to tearing out in the event of an accident, since, according to prior art, the vehicle seat is connected to the underbody via the unit consisting of spring body and sensors so that tensile loads, which occur for example in the event of frontal crashes, are transmitted through the spring body.
Furthermore, resistance strain gauges are not fatigue- and wear-free. They are known to be capable of failing after having been subjected to a certain number of loads. Finally, the resistance strain gauges must be isolated from the surface of the mostly metallic spring body, which requires additional expenditure.